1. Field of the Invention
The present invention is directed to wireless communication systems and, more specifically, to extending a Physical Downlink Control CHannel (PDCCH) from supporting communication in a single cell to supporting communication in multiple cells.
2. Description of the Art
A communication system includes a DownLink (DL) that supports the transmissions of signals from a Base Station (BS) (or Node B) to User Equipments (UEs), and an UpLink (UL) that supports transmissions of signals from UEs to the Node B. A UE, also commonly referred to as a terminal or a mobile station, may be fixed or mobile and may be a wireless device, a cellular phone, a personal computer device, etc. A Node B is generally a fixed station and may also be referred to as a Base Transceiver System (BTS), an access point, or some other similar terminology.
The DL signals include data signals that carry information content, control signals, and Reference Signals (RS), which are also known as pilot signals. The Node B transmits data information to a UE through a Physical Downlink Shared CHannel (PDSCH) and transmits control information to a UE through a PDCCH.
The UL signals also include data signals, control signals, and RSs. A UE transmits data information to the Node B through a Physical Uplink Shared CHannel (PUSCH) and transmits control information through a Physical Uplink Control CHannel (PUCCH). It is also possible for UEs to transmit control information through the PUSCH.
Downlink Control Information (DCI) serves several purposes and is transmitted in DCI formats through the PDCCH. For example, DCI formats are used to provide DL Scheduling Assignments (SAs) for PDSCH receptions by the UEs, UL SAs for PUSCH transmissions by the UEs, or Transmission Power Control (TPC) commands for PUSCH receptions or PUCCH transmissions from the UEs. DCI formats also provide scheduling information for a Paging CHannel (PCH), for a response by the Node B to Random Access CHannels (RACH) transmitted by the UEs, and for Secondary Information Blocks (SIBs) providing broadcast control information from the Node B. The DCI format for transmitting the TPC commands will be referred to as DCI format 3 and the DCI format for transmitting the scheduling information for the transmission of either PCH, RACH response, or SIBs will be referred to as DCI format 1C.
Typically, the PDCCH is a major part of the total DL overhead and directly impacts the achievable DL cell throughput. A conventional method for reducing PDCCH overhead is to scale its size according to the resources required to transmit the DCI formats during a DL Transmission Time Interval (TTI). Assuming Orthogonal Frequency Division Multiple Access (OFDMA) as the DL transmission method, a Control Channel Format Indicator (CCFI) parameter transmitted through the Physical Control Format Indicator CHannel (PCFICH) can be used to indicate the number of OFDM symbols occupied by the PDCCH.
FIG. 1 is a diagram illustrating a structure for the PDCCH transmission in the DL TTI, which for simplicity includes one sub-frame having M OFDM symbols.
Referring to FIG. 1, the PDCCH 120 occupies the first N symbols 110. The remaining M-N symbols of the sub-frame are assumed to be primarily used for PDSCH transmission 130. The PCFICH 140 is transmitted in some sub-carriers, also referred to as Resource Elements (REs), of the first symbol. The PCFICH includes 2 bits indicating a PDCCH size of M=1, M=2, or M=3 OFDM symbols. Additionally, some sub-frame symbols include RS REs, 150 and 160, which are common to all UEs for each of the Node B transmitter antennas, which in FIG. 1 are assumed to be two. The RSs enable a UE to obtain a channel estimate for its DL channel medium and to perform various other measurements and functions. The PDSCH typically occupies the remaining REs.
Additional control channels may be transmitted in the PDCCH region but, for brevity, they are not illustrated in FIG. 1. For example, to support Hybrid Automatic Repeat reQuest (HARQ) for PUSCH transmissions, a Physical Hybrid-HARQ Indicator CHannel (PHICH) may be transmitted by the Node B, in a similar manner as the PCFICH, to indicate to groups of UEs whether or not their previous PUSCH transmission was received by the Node B.
The Node B separately codes and transmits each DCI format through a PDCCH.
FIG. 2 is a block diagram illustrating a conventional processing chain for transmitting a DCI format.
Referring to FIG. 2, the Medium Access Control (MAC) layer IDentity of the UE (or UE ID), for which a DCI format is intended, masks the Cyclic Redundancy Check (CRC) of the DCI format codeword in order to enable the reference UE to identify that the particular DCI format is intended for the reference UE. The CRC 220 of the (non-coded) DCI format bits 210 is computed and is subsequently masked 230 using the eXclusive OR (XOR) operation between CRC bits and the UE ID 240, i.e., XOR(0,0)=0, XOR(0,1)=1, XOR(1,0)=1, and XOR(1,1)=0.
The masked CRC is then appended to the DCI format bits 250, channel coding 260 is performed, for example, using a convolutional code, followed by rate matching 270 to the allocated PDCCH resources, and then interleaving and modulation 280. Thereafter, a control signal 290 is transmitted.
A UE receiver performs the reverse operations of the Node B transmitter to determine whether a DCI format in the PDCCH was intended for the UE.
FIG. 3 is a block diagram illustrating a conventional processing chain for receiving a DCI format.
Referring to FIG. 3, a received control signal, i.e., a PDCCH, 310 is demodulated and the resulting bits are de-interleaved 320. Rate matching applied in the Node B transmitter is restored 330, and the output is subsequently decoded 340. After decoding, the DCI format bits 360 are obtained, after extracting the CRC bits 350, which are then de-masked 370 by applying the XOR operation with the UE ID 380. Thereafter, the UE performs a CRC test 390. If the CRC test passes, the UE considers the DCI format as being valid and determines the parameters for PDSCH reception (DL DCI format) or PUSCH transmission (UL DCI format). If the CRC test does not pass, the UE disregards the DCI format.
The information bits of the DCI format correspond to several Information Elements (IEs) such as, for example, the Resource Allocation (RA) IE indicating the part of the operating BandWidth (BW) allocated to a UE for PDSCH reception or PUSCH transmission, the Modulation and Coding Scheme (MCS) IE, the IE related to the HARQ operation, etc. The BW unit for PDSCH or PUSCH transmissions is assumed to consist of several REs, e.g., 12 REs, and will be referred to as a Physical Resource Block (PRB).
PDCCHs for a UE are not transmitted at fixed and predetermined locations and do not have predetermined coding rates. Consequently, a UE performs multiple PDCCH decoding operations in each sub-frame to determine whether any of the PDCCHs transmitted by the Node B are intended for the UE. In order to assist UEs with the multiple PDCCH decoding operations, the PDCCH REs are grouped into Control Channel Elements (CCEs) in the logical domain. For a given number of DCI format bits as illustrated in FIG. 2, the number of CCEs for the respective PDCCH transmission depends on the channel coding rate. For UEs experiencing low or high Signal-to-Interference and Noise Ratio (SINR) in the DL, the Node B may respectively use a low or high channel coding rate in order to achieve a desired PDCCH BLock Error Rate (BLER). Therefore, a PDCCH transmission to a UE experiencing low DL SINR typically requires more CCEs that a PDCCH transmission to a UE experiencing high DL SINR. Alternatively, different power boosting of CCE REs may also be used in order to achieve a target BLER. Typical CCE aggregation levels for PDCCH transmissions are assumed to follow a “tree-based” structure, for example, 1, 2, 4, and 8 CCEs.
For the PDCCH decoding process, a UE may determine a search space for a candidate PDCCH, after it restores the CCEs in the logical domain, according to a common set of CCEs for all UEs in a UE-Common Search Space (UE-CSS) and according to a UE-specific set of CCEs in a UE-Dedicated Search Space (UE-DSS). The UE-CSS includes the first C CCEs in the logical domain. The UE-DSS may be determined according to a pseudo-random function having UE-common parameters as inputs, such as the sub-frame number or the total number of PDCCH CCEs in the sub-frame, and UE-specific parameters such as the identity assigned to a UE (UE_ID).
For example, for CCE aggregation levels Lε{1,2,4,8}, the CCEs corresponding to PDCCH candidate m can be given by Equation (1).L·{(Yk+m)mod └NCCE,k/L┘}+i  (1)
In Equation (1), NCCE,k is a total number of CCEs in sub-frame k, i=0, . . . , L−1, m=0, . . . , M(L)−1, and M(L) is a number of PDCCH candidates for the respective CCE aggregation levels. Exemplary values of M(L) for Lε{1,2,4,8} are, respectively, {6, 6, 2, 2}. For the UE-CSS, Yk=0. For the UE-DSS, Yk=(A·Yk-1)modD where, for example, Y−1=UE_ID≠0, A=39827 and D=65537.
DCI formats conveying information to multiple UEs, such as DCI format 3 or DCI format 1C, are transmitted in the UE-CSS. If enough CCEs remain after transmitting DCI formats 3 and 1C, the UE-CSS may also convey some DCI formats for PDSCH receptions or PUSCH transmissions by UEs. The UE-DSS exclusively conveys DCI formats for PDSCH receptions or PUSCH transmissions. In an exemplary setup, the UE-CSS includes 16 CCEs and supports 2 PDCCH with L=8 CCEs, or 4 PDCCH with L=4 CCEs, or 1 PDCCH with L=8 CCEs and 2 PDCCH with L=4 CCEs. The CCEs for the UE-CSS are placed first in the logical domain (prior to interleaving).
FIG. 4 illustrates a conventional PDCCH transmission process. After channel coding and rate matching, as illustrated in FIG. 2, the encoded DCI format bits are mapped to CCEs in the logical domain.
Referring to FIG. 4, the first 4 CCEs (L=4), CCE1 401, CCE2 402, CCE3 403, and CCE4 404 are used for DCI format transmission to UE 1. The next 2 CCEs (L=2), CCE5 411 and CCE6 412, are used for DCI format transmission to UE2. The next 2 CCEs (L=2), CCE7 421 and CCE8 422, are used for DCI format transmission to UE3. The last CCE (L=1), CCE9 431, is used for DCI format transmission to UE4.
The DCI format bits may be scrambled 440 using a binary scrambling code, which is typically cell-specific, and are subsequently modulated 450. Each CCE is further divided into mini-CCEs. For example, a CCE including 36 REs can be divided into 9 mini-CCEs, each having 4 REs.
Interleaving 460 is applied among mini-CCEs (blocks of 4 QPSK symbols). For example, a block interleaver may be used where the interleaving is performed on symbol-quadruplets (4 Quadrature Phase Shift Keying (QPSK) symbols corresponding to the 4 REs of a mini-CCE) instead of on individual bits. After interleaving the mini-CCEs, the resulting series of QPSK symbols may be shifted by J symbols 470, and then each QPSK symbol is mapped to an RE 480 in the PDCCH region of the DL sub-frame. Therefore, in addition to the RS from the Node B transmitter antennas, 491 and 492, and other control channels such as the PCFICH 493 and the PHICH (not shown), the REs in the PDCCH include QPSK symbols corresponding to DCI format for UE1 494, UE2 495, UE3 496, and UE4 497.
In order to support higher data rates and signal transmission in BWs larger than the BWs of individual carriers (or cells) supporting legacy communications, aggregation of multiple carriers (or cells) can be used. For example, to support communication over 100 MHz, aggregation of five 20 MHz carriers (or cells) can be used. For ease of description, UEs that can only operate over a single carrier (or cell) will be referred to herein as Legacy-UEs (L-UEs) while UEs that can operate over multiple carriers (or cells) will be referred to herein as Advanced-UEs (A-UEs).
FIG. 5 illustrates a principle of carrier aggregation. An operating BW of 100 MHz includes the aggregation of 5 (contiguous, for simplicity) carriers, 521, 522, 523, 524, and 525, each having a BW of 20 MHz. Similarly to the sub-frame structure for communication over a single carrier in FIG. 1, the sub-frame structure for communication over multiple carriers includes a PDCCH region, for example, 531 through 535, and a PDSCH region, for example, 541 and 545.
FIG. 6 is a diagram illustrating a conventional heterogeneous network deployment.
Referring to FIG. 6, an area covered by a macro-Node B 610 encompasses areas covered by micro-Node Bs 620 and 630. Because the macro-Node B covers a larger area than a micro-Node B, its transmission power is substantially larger than the transmission power of a micro-Node B. Consequently, for topologies such as illustrated in FIG. 6, the signals transmitted by a macro-Node B can cause severe interference to the signals transmitted by a micro-Node B. Interference coordination techniques can be applied to PDSCH transmissions to mitigate macro-to-micro interference using different PRBs between PDSCH signal transmissions from the macro-Node B and a micro-Node B. However, such interference coordination is not possible for the PDCCH because the CCEs are pseudo-randomly distributed over the entire operating BW, as was previously described.
To avoid interference to PDCCH transmissions in a micro-cell, all PDCCH transmissions can be in the macro-cell and a Carrier Indicator, or Cell Indicator, (CI) IE can be introduced in the DCI formats to indicate whether the DCI format is for the macro-cell or for the micro-cell. For example, a CI IE of 2 bits can indicate whether the DCI format is for the macro-cell or for any of a maximum of three micro-cells.
In addition to providing PDCCH interference avoidance, PDCCH transmission in certain cells may be avoided for practical reasons. For example, it is desirable to avoid PDCCH transmissions in cells with small BW as they are inefficient and lead to large respective overhead. Also, PDSCH transmissions in a cell can be optimized to occur over all DL sub-frame symbols if transmissions of PDCCH and of other supporting signals such as UE-common RS, are avoided.
The CI functionality can accommodate:
PUSCH scheduling in the UL of multiple cells through PDCCH transmission in a single cell;
PDSCH scheduling in the DL of multiple cells through PDCCH transmission in a single cell; and
PDCCH transmission in a first cell (macro-cell) and in a second cell (micro-cell).
FIG. 7 is a diagram illustrating a conventional PUSCH scheduling in the UL of multiple cells through PDCCH transmission in a single cell.
Referring to FIG. 7, a PDCCH in a single cell 710 is associated with the UL of two cells, 720 and 730. Consequently, PDCCHs scheduling PUSCH transmissions from Cell 1 and Cell 2 are transmitted in a single cell and the cell of PUSCH transmission can be identified by a CI IE consisting of 1 bit.
FIG. 8 is a diagram illustrating a conventional PDSCH scheduling in a DL of multiple cells through PDCCH transmission in a single cell.
Referring to FIG. 8, only Cell1 810 and Cell3 830 transmit PDCCH. Scheduling for Cell2 820 is performed through PDCCH transmission in Cell1 810 and scheduling for Cell4 840 and Cell5 850 is performed through PDCCH transmissions in Cell3 830.
FIG. 9 is a diagram illustrating a conventional PDCCH transmission in a first cell (macro-cell) and in a second cell (micro-cell), which may occur to avoid interference in PDCCH transmissions between a macro-cell and a micro-cell.
Referring to FIG. 9, although both macro-cell and micro-cell may have PDSCH transmissions in Cell1 910 and Cell2 920, the macro-cell transmits PDCCH only in Cell1 910 and the micro-cell transmit PDCCH only in Cell2 920.
One issue for supporting PDCCH transmissions using a CI is the PDCCH size. In communication systems having a single cell, the PDCCH is assumed to be limited to a maximum number of M OFDM symbols. In communication systems having multiple cells and having PDCCH transmission in a single cell, this limitation of the PDCCH size may cause scheduling restrictions. In general, the PDCCH size may need to be increased if the PDCCH in one cell performs scheduling in multiple cells.
For the UE-CSS, which is assumed to include a fixed number of CCEs, it may not be possible to transmit additional PDCCH corresponding to additional cells.
For the UE-DSS, modification and expansion is needed in order to transmit multiple DCI formats to a UE in the PDCCH region of a single cell.
For the blind decoding operations a UE needs to perform, their number may scale linearly with the number of cells for which PDCCH is transmitted in a single cell. It is desirable to avoid such an increase in order to avoid the associated impact on the UE receiver complexity.
Therefore, there is a need to expand the PDCCH region in a single cell to support PDCCH transmissions for scheduling in multiple cells.
There is a further need to expand the UE-CSS in a single cell to enable PDCCH transmission conveying UE-common information for multiple cells.
There is another need to expand the capacity of the UE-DSS in a single cell for scheduling over multiple cells.
Additionally, there is another need to reduce the number of blind decoding operations a UE needs to perform.